There are currently thousands of wells for extracting oil, both on land and under the sea, the latter exploited thanks to suitable platforms where extraction is controlled remotely and where the oil is collected and possibly treated before being transferred to mainland.
The composition of the extracted oil can vary with the age of the deposit: in addition to natural gas, a variable amount of water is generally present, and it increases as the age of the well increases. It is generally at the surface that gas and water are separated from the oil once extracted, by means of “triphasic” separators, i.e. capable of separating the three phases present in the mixture—oil, water and gas—then gas and water are collected and possibly re-injected under pressure into the reservoir to maintain its high productivity.
Conventional triphasic separators installed at the surface, on mainland in land reservoirs or on a platform in the case of subsea reservoirs, are bulky apparatuses, generally consisting of horizontal cylindrical containers of great size, with which it is possible indeed to rid the crude oil of gas and water before exportation from the plant for subsequent commercialisation.
Conventional triphasic separators are generally gravity separators, which take advantage of the different density of the three components to separate the mixture: in practice, the denser water phase is left to form a layer on the bottom of the container and the less dense oil phase forms a layer above it, whereas the gas dispersed in the mixture rises to the area above the two liquid layers. In addition to the size, these apparatuses also have the drawback of having a very low separation efficiency.
A system for the separation of two liquids having different densities, which may also contain a gas, is disclosed in European patent application No. EP0955076A1. This system is based on the use of a cylindrical separator with an inlet for the mixture to be separated in the upper part where a first gas-liquid separation occurs: the gas exits from a nozzle positioned in the upper part of the side walls while the liquid-liquid mixture stratifies below the upper part of the separator. A vertical duct 11 is positioned so as to put in communication the upper part with a lower part of the separator, and includes packings structures. The passage through these structures promotes a first separation of the two liquids by coalescence. The use of such structures is limited, for instance by the presence of solid residues in the mixture, which may cause a block. Moreover, in this system the upper and the lower parts are separated between each other, not allowing the fluid communication between the separated less dense liquid phase and the gas and consequently the release of the gas possibly trapped in the liquid phase; the gas is therefore dragged within the liquid phase.
The International patent application No. WO 2011/054192 discloses a partial separator of water from a triphasic stream water-oil-gas having a high content of water. This vertical separator has a cylindrical configuration suitable to be mounted on oil pipes for reducing the water cut of the multiphase stream. In one of the embodiment disclosed in WO 2011/054192 the separator is divided in an upper and a lower part: the upper part, where the gas-liquid separation occurs, is provided with the inlet for the stream and is connected to the lower part by means of a vertical pipe provided with a float valve which discontinuously regulates the stream of the water-oil mixture in the lower part of the separator; here part of the water in the water-oil mixture is separated by gravity. This system only carries out a partial separation of the triphasic mixture and for its operating modes it is suitable mainly on land or on platforms.
In the case of subsea reservoirs consisting of a plurality of wells, in industrial applications a single separator is usually fed by many wells. In this case, the separator is necessarily very large, also due to the possible presence of liquid slugs in the supply, where by the term “slug” are indicated alternating blocks of liquids and gas coming out from the reservoir. As can easily be understood, separators of this type are difficult both to install and to manage, and their potential collapse can constitute an unsustainable environmental and economic risk.
In British patent No. GB2394737 a modular system is disclosed for the separation of phases in a multiphase stream, suitable for subsea applications. A separation system is disclosed in this patent where the fluid to be separated is selectively conducted to at least a first gravity separator and at least a second gravity separator in parallel or in subsequent steps depending on the properties of the fluid and process conditions, in combination with other separation systems such as hydrocyclones or electrostatic coalescers. The presence of such systems allows the upstream separation of the gaseous phase and the consequent simplification in controlling the modular separation system, but it makes the overall system much less reliable in terms of maintenance and frequency of breakages.
Therefore, there is still a need for an apparatus allowing triphasic separation that is efficient, compact, low in cost and that can be used on the surface, but also and particularly on the seabed in the case of subsea reservoirs. The advantage of having the separator on the seabed is substantial, since it avoids for instance bringing to the surface the water and oil mixture, being possible to re-inject the water produced in a suitable subsea well dedicated to the re-injection of water.